This invention relates generally to bumpers and, more particularly, to energy absorbing vehicle bumper systems.
Bumpers typically extend widthwise across the front and rear of a vehicle and are mounted to rails that extend in a lengthwise direction. Energy absorbing bumper systems attempt to reduce vehicle damage as a result of a collision by managing impact energy and intrusion while not exceeding a rail load limit of the vehicle. The efficiency of an energy absorber is defined as the amount of energy absorbed over distance. A high efficiency energy absorber absorbs more energy over a shorter distance than a low energy absorber. High efficiency is achieved by building load quickly to just under the rail load limit and maintaining that load constant until the impact energy has been dissipated.
Increasing the stiffness of an energy absorber generally increases the efficiency of the absorber since a stiff energy absorber builds load more quickly than a less stiff absorber. In addition, there generally is less intrusion with a stiff energy absorber than with a less stiff energy absorber. Increasing the stiffness of the energy absorber, however, can result in an undesirable increase in the weight of the bumper system. Specifically, a more stiff energy absorber generally is heavier than a less stiff absorber.
Some known energy absorbing bumper systems include a foam resin, such as described in U.S. Pat. No. 4,762,352 and U.S. Pat. No. 4,941,701. Foam based systems typically have slow loading upon impact, which results in a high displacement. Further, foams are effective to a sixty or seventy percent compression, and beyond that point, foams become incompressible so that the impact energy is not fully absorbed. The remaining impact energy is absorbed through deformation of a backup beam and/or vehicle structure. Foams are also temperature sensitive so that displacement and impact absorption behavior can change substantially with temperature. Typically, as temperature is lowered, foam becomes more rigid, resulting in higher loads. Conversely, as temperature rises, foams become more compliant resulting in higher displacements and possible vehicle damage.
Some known bumper systems include crash cans. The crash cans are separately fabricated and attached directly to the beam in alignment with the vehicle rails. The crash cans absorb energy during impact, e.g., an offset impact, and facilitate preventing damage to the beam. Separately fabricating and attaching the crash cans to the beam, however, increases bumper assembly costs and complexity.